The present invention relates generally to signal processing, and more specifically to techniques for canceling acoustic echo using channel control and post filtering.
Full-duplex hands-free communication systems are used for many applications, such as speakerphone, hands-free car kit, teleconferencing system, cellular phone, and so on. For each of these systems, a microphone picks up an acoustic signal emitted by a speaker and the reflections from the borders of an enclosure, such as a room or a car. In the case of a telecommunication system, users are often annoyed by listening to their own voice, which is delayed by the path of the system. This acoustic disturbance is referred to as echo.
Echo cancellation is often required in many communication systems to eliminate echo as well as remove howling effect. For example, echo cancellation is typically used in a hands-free full-duplex environment, such as a vehicle or a room, where the speaker and microphone may be located some distance away from a user. Conventionally, echo cancellation is achieved by a circuit that employs an adaptive filter. This adaptive filter may implement a least mean square (LMS) algorithm or a normalized least mean square (NLMS) algorithm. The adaptive filter performs echo cancellation based on a reference signal, which may be a line input from a communication or telematics device such as a cellular phone or some other device. The adaptive filter is typically able to remove the portion of the echo that is correlated to the reference signal.
However, conventional echo cancellation techniques are not able to remove certain portion of the echo. For example, non-linearity of the circuitry in the communication systems (e.g., the speaker, analog-to-digital (A/D) converter, digital-to-analog (D/A) converter, and so on) generates echo that is not correlated to the reference signal. This type of echo cannot be canceled by conventional echo cancellation techniques which only employ an adaptive filter. Moreover, user movement and volume change can cause the echo path to vary. This results in varying echo that typically cannot be canceled very well, particularly if the echo path changes faster than the convergence rate of the adaptive filter. For these and other reasons, a portion of the echo often remains after the conventional echo cancellation with the adaptive filter.
Non-linear echo cancellation techniques may be used to attempt to cancel the remaining echo. However, some conventional non-linear echo cancellation techniques, such as a center clipper, can cause voice distortion by cutting off low power voice. The techniques also cannot handle high volume echo. Conventional center clippers are described, for example, in U.S. Pat. Nos. 4,031,338, 4,679,230 and 5,475,731, and European Patent Nos. EP-0164159-A1 and EP-0164159-B1. Other conventional non-linear echo cancellation techniques, such as conventional post filters, also cannot deal with very strong echo and serious non-linearity.
As can be seen, techniques that can effectively cancel acoustic echo in communication systems are highly desirable.